The intent of using GPLv3 was to avoid people private-forking the plugin and using it as a framework for their non-free products to be sold.
You see, 90% of the value of ReadonlyREST is decoupling the ever-changing and opaque internal ES API and keeping a consistent interface for rules. This is an ideal framework for anyone to build a lot of cool things on ES.
And they are welcome to do so, as long as it’s PR’d back to this project or forked into another equally “libre” project.
If someone wants to do a proprietary product on ReadonlyREST framework, I’m ready to re-license it with a commercial license to whomever wants, but certainly in a VERY controlled fashion.
Now, if ReadonlyREST was Apache licensed, people would add it as a library and sell their derived closed source product.
Does GPL create any other sort of issues at work? If you use it “as is” in your enterprise, you should have no problem (it’s like using Linux in your servers where proprietary software runs).